My version of Inurin, inspired by the information in "Pirates of Drinax" and the wiki article, but slightly different:

History

Inurin was settled in the year -1844, during the Second Imperium. The colonists arrived in one large wave, and built settlements near strategic resources on the main continent of the planet. Those settlements later evolved into independent city states. The colonists had planned to be largely self sufficient and therefore accounted for a certain drop in tech level, and managed to build up an industrial base with that in mind. Therefore, when the Long Night came, it wasn't really catastrophic.

More problematic was the native life on Inurin. The biosphere of Inurin is very active and similar to Earth, and numerous pathogens managed to cross over to humans, resulting in waves of plagues that swept over the planet. Those occasional plagues remained a problem for a long time and greatly limited the progress Inurin could make.

Beginning in the mid-500s, visitors from the stars became more frequent once again, but apart from agricultural products, Inurin had not much to offer, so was largely ignored.

Around 1000, an isolationist religious splinter group managed to take power over the Southern city states on Inurin. Only a coalition of all the other city states managed to defeat them (the “Coalition Wars”). The survivors were offered the choice to renunciate their views or leave. A few hundred thousand chose exile and now live on a separate island.

The war took a huge death toll, but also brought technological progress. The city states started to produce hydrogen and advertise to starships, and without a common enemy, the Coalition broke once again and lead to rivalry and even an arms race. But offworld commerce, even as limited as it was, also brought access to higher TL medicine and vaccines, resulting in a huge rise in population.

During the Coalition Wars, banks and corporations cooperated more closely, and parts of the elite started to see the bigger picture. Lobby groups convinced the two biggest states to form a “New Coalition” to present a unified front towards other planets and starfarers, and the other states were convinced to join. In 1086, the “Planetary Council of Inurin” formed a representative government of the whole planet (even the isolationist sectarians were forced to formally join), and the “Interplanetary Council of Inurin” reached out to the Third Imperium and began to formalize relationships with neighboring star systems.

In 1103, the Interplanetary Council of Inurin secretly established a mining outpost on Ergo, and the profits from it are exclusively spent to expand its power base.

Political situation

People on Inurin live in city states. The old colonies form the centers, and towns and villages spread out from them. Details vary, but they are all governed my some form of representative democracy. The only exception to this is Itaka: Formed after the Coalition Wars by exiled religious fanatics, it is a new city, and is ruled by a theocracy.

All city states send representatives to the Planetary Council of Inurin that forms an umbrella government. It's power is limited, but they regulate offworld commerce and distribute the profits from refueling starships and interstellar trade.

The Interplanetary Council of Inurin is not democratically legitimized and consists of influental Inurians from different groups. They don't meddle in internal affairs (although they have a certain influence on the Planetary Council), and promote their interstellar agenda.

Religion

The first colonists agreed to adhere to one religion. They decided on some form of Unitarism, with a vague believe in monotheism/pantheism, but without spelling out any details, and with a strong emphasis on community on one hand, and tolerance and search for individual spirituality on the other hand.
This religion, the “Church of One”, still exists today and plays a major role in everyday life, but it has grown an appendix over time. There is a mystery cult with secret rituals and different levels of initiation, formed around the consumption of Sarna, a psychoactive substance harvested from indigenous fungi. Sarna induces spiritual experiences of varying level. Most adults on Inurin are at least on the first level of initiation, which is seen as a rite of passage into adulthood.